Noise control is a primary concern for the transmission design engineer, and particularly so in the design of helicopter main transmission gearboxes. Excessive vibrations generated by transmission gearboxes typically result in undesirable noise levels in helicopter cockpits and/or cabins, which causes operator/passenger aural discomfort. Cabin and/or cockpit noise abatement is a particular concern in helicopters wherein the final stage of reduction gearing of the main transmission comprises one or more bull pinions interacting with a central bull gear (as opposed, for example, to helicopter main transmissions wherein the final stage of reduction gearing comprises epicyclic gearing).
For example, Sikorsky helicopters of the S-76.RTM. (S-76 is a registered trademark of the Sikorsky Aircraft Division of United Technologies Corporation) series, e.g., S-76A, S-76B, S-76C, have a main transmission that includes three stages of reduction gearing: a first stage for each engine output consisting of helical gearing, an intermediate stage consisting of spiral bevel gearing, and a final reduction stage comprising a central bull gear that intermeshes with right and left hand bull pinions (to combine the inputs of the two engines that provide the motive power for S-76.RTM. helicopters). Research has shown that the cockpit and/or cabin noise levels of S-76.RTM. helicopters are primarily the result of vibrations originating in the main transmission gearbox.
A-weighted octave levels and overall DBA levels recorded in the cockpits and/or cabins of S-76A, S-76B, and S-76C helicopters indicate that interior noise levels are predominately the result of vibrations occurring at the bull gearing meshing frequency of 778 Hz, as illustrated in FIG. 1. The vibrations produced by the first and second reduction stages of S-76.RTM. main transmission gearboxes, i.e., the noise levels generated by the helical and spiral gearing as illustrated in FIG. 1, occur at higher frequencies and typically are not significant relative to the dominant noise levels produced by the fundamental and first few harmonics of the bull gearing meshing vibrations.
The low frequency gearbox vibrations resulting from bull gearing meshing are coupled into the helicopter airframe via the transmission gearbox housing. The resultant airframe vibrations generate noise in the helicopter cockpit and/or cabin. Such low frequency noise generally cannot be effectively abated by acoustic treatment of the cockpit and/or cabin interior, and therefore, effective noise control solutions must be implemented at the noise source, i.e., the main transmission gearbox.
To effectively abate such low frequency noise, it is necessary to identify the primary causal factor(s) of bull gearing vibrations. The vibrations generated by the aforedescribed helicopter main transmission gearboxes are produced predominately by meshing between misaligned bull gearing, i.e., the central bull gear and bull pinion(s). FIG. 2 is a schematic representation of an exemplary three stage main transmission that depicts the meshing interaction between the central bull gear and two bull pinions. FIG. 2A illustrates the idealized intermeshing relationship between the central bull gear (illustrated by full lines) and a bull pinion (illustrated by dashed lines) wherein respective bull gear and bull pinion teeth engage completely and simultaneously across corresponding faces.
Previous efforts to reduce the noise levels generated by intermeshing between misaligned bull pinions and the central bull gear included lead correction modifications and modifications in effective bull gear tip relief. While such modifications resulted in some reduction in bull gearing vibrations, the resultant reduced interior noise levels of S-76.RTM. helicopters were adjudged to still present an unacceptable level of aural discomfort.
A need exists to more fully identify the design and operating parameters that cause main transmission gearbox vibrations, and in particular, S-76.RTM. main transmission gearbox vibrations. An effective solution for noise abatement can only be implemented based upon the identification and understanding of the causal parameter(s) of main transmission gearbox vibration generation.